Stoichiometries electropolymerization

A number of investigators have reported the detection of protons released from the de-prontonation step of coupled products during the electropolymerization. Bruckenstein and Sharkey [76] observed protons released during the polymerization reaction of PyH and azulene using rotating ring-disk electrodes. A similar observation was made by Beck and Oberst [77], who monitored the proton/electron stoichiometry, the accuracy of which was found to be limited by the occlusion of acid in the pores of a growing polymer film. 

Electrochemical polymerization reactions have electrochemical stoichiometry and in this regard are different from traditional polymerization reactions which are initiated either directly or indirectly [4], and which take place away from the electrode surface. In addition, the product of the electropolymerization reaction produces a film which has electroactivity and electrical conductivity [7], in contrast to many other organic electrosynthesis reactions where the electrode is covered with a product film which passivates the electrode. Moreover, many of the films are easily prepared from commercially available reagents, are stable and show little degradation in their electrical and mechanical properties in an ambient atmosphere. 

Potential Electropolymerization is carried out at moderate potentials to prevent the oxidative decomposition of the solvent, electrolyte and polymer film. The polymerization potential also determines the stability of intermediate species. The formation of a polypyrrole film, for example, occurs via cation intermediates whose stability favours the radical coupling reaction. The reactive cations may also react with solvent and other nucleophiles in the vicinity of the electrode surface, minimizing the polymer forming reaction. Some of the monomers which have been electropolymerized are listed in Table 2.3 along with their respective peak potentials and the apparent electrochemical stoichiometry of the reaction. 

In comparatively rare cases reductive electropolymerization can also be used. In this case the monomers usually contain halogen substituents at the positions where the coupling takes place. In contrast to oxidative coupling, some catalyst is added to enhance the coupling process. Figure 7b shows the stoichiometry of reductive coupling. Reductive electropolymerization has been used for the preparation of certain thiophene and pyridine derivatives. 

From a mechanistic point of view, the most important feature of pyrrole electropolymerization (on inert electrodes and at low potentials where no parallel reactions coexist) is that it proceeds through an electrochemical stoichiometry, with n values in the range 2.0-2.7 Faraday/mol of reacting monomer [45-7]. From elementary analysis it has been deduced that the film-forming process needs 2 Faraday/mol, that it, 2 electrons per molecule. The excess of charge corre-... 

---